Thailand's Constitutional Court has ruled that a general election held in February was invalid, setting the stage for a new vote and dealing another complication to the country's political crisis.

The judges voted 6-3 on Friday to declare the February 2 election unconstitutional because voting was not held that day in 28 constituencies where anti-government protesters had prevented candidates registering.

The constitution says the election must be held on the same day nationwide.

"The process (now) is to have a new general election," Pimol Thampitakpong, the court's secretary-general, said at a news conference.

There was no immediate indication of when new polls might be held. The date is usually set by the government in consultation with the election commission.

The ruling would appear to have little practical effect in either alleviating or worsening Thailand's political crisis, which began in late 2013 when protesters demanded the administration of Prime Minister Yingluck Shinawatra be replaced by an unelected "people's council" to implement reforms they say are needed to end corruption and money politics.

Yingluck refused to resign and called early elections in a bid to ensure a fresh mandate. But the protesters tried to prevent the election taking place, physically blocking and intimidating potential candidates and voters.

It was their efforts that prevented voting being completed on the same day.

At the same time, the main opposition Democrat Party - closely linked to the protest movement - boycotted the polls. Because voting was never completed, no results were announced, even for areas where there were no problems.

The Democrats indicated earlier this week they would boycott fresh polls if held under Yingluck's caretaker government.

Even if new polls go smoothly, Yingluck faces several legal challenges that could force her from office, faced with a judiciary that has a record of hostility towards her and her political allies.

The protesters, whose main strength is in the Democrats' southern strongholds and Bangkok, have maintained constant, sometimes violent street demonstrations in the capital. In turn, they have been the target of gun and grenade attacks by unknown parties.

The attacks, along with street battles against the police and political rivals, have left at least 23 people dead and hundreds hurt.